The influence of the autonomic nervous system (ANS) has been implicated in multiple cardiac pathophysiologic processes. In normal sinus rhythm, analysis of heart rate variability (HRV) is a useful tool to investigate these influences. It has permitted the assessment of parasympathetic, and to some extent sympathetic, influences in a wide variety of circumstances, and has facilitated the investigation of the interaction of heart failure and the ANS, as well as the effects of cardioactive drugs on neurohumoral balance. Depressed HRV has been demonstrated to predict long-term risk arrhythmic death after MI, and suggestive data is emerging that HRV may provide independent prognostic information in patients with nonischemic cardiomyopathies.